Variation Order
A formal instruction changing the scope, quality or sequence of the works, issued under the contract’s variations clause. It is the mechanism by which the contract accommodates change without either party breaching it.
The instrument is not the problem. The problem is everything that changes the work without one — the drawing revision that quietly adds scope, the verbal instruction on a Friday, the "while you are there, could you also". These are constructive variations, and they are variations whether or not anybody issued a document.
The practical discipline is to convert them into instruments immediately. A contractor who does the extra work and raises the paperwork later is negotiating from the worst position available: the work is done, the leverage is gone, and the conversation is now about whether it was really extra.